6 HVAC Smells and What They Mean

Woman holding her nose because of bad smell from AC

Things that don’t pass the smell test: your dog’s innocent eyes following a missing bag of donuts, your child’s tall tale of how the crayon wall art came to be, and the actual air you breathe in your home. The latter, more literal, occurrence? That’s most likely caused by something amiss in your AC unit or heating system. Below, we’ll take a look at six HVAC smells and what they mean — from rotten eggs to dirty socks to mustiness and more.

But First, What Is an HVAC System?

hvac heating and condensing units
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An acronym for heating, ventilation, and air conditioning, an HVAC system combines all of those elements to provide homeowners with comfortable living spaces and better air quality. Other HVAC features include:

  • thermostats
  • air ducts
  • air handlers
  • condenser and evaporator coils
  • compressors

6 HVAC Smells and What They Mean

1. Dirty Sock Smell

Getting a whiff of something that smells like dirty socks? It’s probably coming from your air conditioner and likely signals bacterial development on the evaporator coils of your indoor unit. Other reasons for that bad smell include dirty air filters or a drain pan that’s missed a few cleanings.

If washing those components doesn’t remedy the problem, have an HVAC technician check things out. Not only will leaving that scent to fester cause an unpleasant living environment, but it can also trigger allergies, and potentially, other illnesses.

2. Rotten Egg Smell

Since natural gas has no smell but can be dangerous if undetected in your home — it can cause fire or even suffocation, if the gas overtakes the amount of oxygen in the air — HVAC techs typically add sulfur to the gas. That way, if there’s a leak, homeowners will know something’s wrong right away.

If this sulfuric, rotten egg smell is the foul odor you’re experiencing, contact your local fire department immediately, shut off the gas to your home, and head outside while you wait for help to arrive. Be sure to call your local HVAC professional, too. Gas leaks of any kind are never something to play with.

3. Musty Smell

This AC smell is most often caused by mold spores and mildew that have proliferated throughout your ductwork. Because a buildup of excess moisture, coupled with a lack of airflow, can occur when your air conditioning system is not in use, it’s important to keep it running on warm days. 

In addition to cooling your home, your AC unit doubles as a dehumidifier, keeping any surplus dampness at bay. Whether you’re going away on vacation for a week or more, or your home will be sitting empty on the market, avoid turning your air conditioning unit off — you may end up with a more serious problem than just HVAC smells on your hands.

Other common causes of mustiness include:

  • Dirty air filters/vents
  • Dirty evaporator coils
  • AC unit is wrong size for your home
  • Clogged drain lines

4. Dead Animal Smell

If, upon turning on your heat pump or air conditioner, you notice the stench of decay, it may mean a pest or two sought shelter in your HVAC unit … and then died. As the vermin begins to decompose in your ductwork, your blower is circulating the cooled or warmed air from said ductwork into your living spaces. A pest control expert can fix this situation for you.

5. Burning Smell

This signifies an electrical odor and likely means your HVAC is working too hard, and thus, overheating. Specifically speaking, the burning smell may be attributed to an electrical problem like faulty wiring or a malfunctioning AC fan or compressor (the part that transforms refrigerant from vapor to gas, depending on the step in the cooling process). 

Improper electrical connections or installations may also cause a burning smell. As ignored electrical problems can potentially cause a fire to start, it’s important to have a problem such as this looked at right away.

6. Sewage Smell

For HVAC systems with sewage lines located nearby, there’s a possibility a pipe could crack, leaking a myriad of sewage gunk into the surrounding ground. And the smell of that sewage? It may find its way into your vents, and consequently, into the air you breathe. As well as smelling horribly, sewage gives off methane gas, which is harmful to your health. Call a plumber to take care of this right away. 

Other reasons you may be smelling something akin to sewage? Dirty air filters or clogged drain lines. Air filters collect pollutants, and over time, if not changed, those pollutants can begin to stink. The buildup in clogged drain lines can also start to smell and that smell can seep into your home.

FAQ About HVAC Smells

Can you smell a refrigerant leak?

Yes. Apparently, refrigerant is said to smell sweet, kind of like chloroform. If you notice this type of smell in the air of your home, call an HVAC service provider to inspect the cause. Leaking refrigerant not only compromises the effectiveness of your HVAC and contributes to climate change, but breathing it in for prolonged periods can also cause lung damage, brain damage, and death.

What should you do to prevent bad smells in your HVAC?

The best way to ensure your HVAC unit doesn’t start emitting foul odors is to maintain regular HVAC tune-ups, which involve cleaning components and checking for problems before they get out of hand. In the meantime, though, you can hold bad smells at bay by changing air filters, clearing the drain line, and cleaning your air conditioning coils.

What is “dirty sock syndrome”?

Earlier, we discussed what to do when your AC system smells like dirty socks. But “dirty sock syndrome,” specifically, tends to pertain to that slightly off odor you smell after turning on your air conditioner for the first time every spring. It’s because following several months of unuse, mold, mildew, dust and other pollutants have had a field day multiplying on or near your AC’s coils. Clean them yourself, or call in an expert, and that should take care of things.

When to Hire a Professional

Troubleshooting a musty or dirty sock smell is something you can do on your own, as it usually only requires cleaning coils, clearing the drain line, or changing air filters. But getting rid of a dead animal in your ductwork or tackling more dangerous scents like rotten eggs, electrical odors, or sewage will require help from an HVAC pro near you.

They’ll locate and identify the problem and perform any necessary repairs or replacements for your system. Then, once again, your home will be bad smell- and danger-free.

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Andréa Butler

Andréa Butler is a writer and editor. And while she hasn't been blessed with DIY skills herself, she is adept at writing and enjoys sharing home improvement tips and pool care guides for the true DIYers out there.